The invention relates generally to a method for making steel articles having a core which contains pearlite and an outer surface of martensite.
In order to produce high-strength wires for cables, carbon-rich steel wires are cold-drawn by passing them through a series of dies of continuously decreasing size. As a starting material, it is requisite to use wires which possess not only high strength but which also have good cold-working characteristics.
The high strength of the wires is necessary so that the forces generated during the drawing operation may be transmitted along the wire whereas the good cold-deformability is intended to prevent cracking or tearing during the drawing operation. These necessary characteristics were, according to the prior art, achievable only when the microstructure of the entire steel wire consisted of fine pearlite with no more than small amounts of ferrite segragation. Such microstructure could only be produced when the wire, which was normally hot-rolled prior to being cold-drawn, was subjected to an additional heat treatment after being cooled to room temperature. This heat treatment is the so-called patenting treatment. Without it the hot-rolled wire, was not well-suited for an immediately following cold-deformation, and could be subject only to relatively low reductions in area, that is, between 35 and 50 percent.
The patenting treatment usually is applied to wires having dimensions smaller than 13 millimeters and mostly consists of reheating the wires into the austenitic range and then rapidly cooling the wires to a temperature below 500.degree. C. in a lead bath or salt bath. The wire is held at this temperature for a period of at least 10 seconds. All of the carbon in the wires is in solution in the austenite retained during the cooling and, by virtue of the isothermal transformation of the retained austenite at this temperature, there is obtained fine pearlite having outstanding drawing characteristics.
Although the patenting treatment leads to an improvement in the drawing characteristics of the wires, there has been a problem associated with the use of this treatment. This resides in the fact that the patenting arrangements using lead or salt baths have a relatively low capacity of 1.8 to 3 tons per hour whereas the continuous wire rolling mills have a capacity of over 40 tons per hour. Thus, it has not been possible to effectively integrate these patenting arrangements with the rolling procedure.
A direct patenting of steel wire making use of the heating required for hot-rolling has already been proposed some time ago in the U.S. Pat. Nos. 1,232,014 and 1,295,139. However, it has only been quite recently that three arrangements have been developed which are practical to use and which have been introduced into service. These arrangements are manufactured, respectively, by the Stelmore Company of Canada, the Demag-Yawata Company of Germany and the Schloemann Company of Germany.
All three arrangements have in common that the wire is precooled in a water quenching stage after leaving the last stage of the hot-rolling mill and is subsequently cooled to a temperature below the transformation temperature for fine pearlite. In these arrangements, the wire is cooled to below the transformation temperature in extended form rather than first being wound into the form of a coil as is conventional. It is only after being so cooled that the individual windings are assembled and shaped into the form of rings in a coil assembly station. The formation of fine pearlite is achieved with each of the three arrangements. On the other hand, all of the arrangements possess the disadvantage that high capital investment costs are associated therewith.
A method of producing wire wherein the wire is wound into the form of a coil and then cooled is proposed in the U.S. Pat. No. 2,756,169. Here, the wire leaves the last hot-rolling stage at a temperature of 982.degree. C. and is cooled to a temperature between 482.degree. and 750.degree. C. in a period of 1.5 seconds by being passed through a multi-step water quenching stage. The wire is then coiled and thereafter cooled to room temperature in air. During the air-cooling the heat loss resulting from the cooling is, for a short period of time, compensated for by the heat of transformation which is released so that the wire undergoes a substantially isothermal transformation to pearlite. In this method, which has strong similarity to the lead bath patenting procedure, it is, however, recommended that the water-cooling not drive the temperature of the wire below 482.degree. C. This is based on the fact that cooling of the wire to below this temperature results in an intermediate stage, namely, a martensitic transformation. The reason presented for not cooling the wire to below 482.degree. C. is that this would again substantially worsen the drawing characteristics of the wire. In any event, the method according to the U.S. Pat. No. 2,756,169 has a certain disadvantage associated with it. This resides in that, for the rolling speeds conventionally used and, in particular, for the rolling speeds which it is attempted to achieve in the future, the multi-step cooling of the wire while it is in extended or linear form in the manner proposed in this patent results in quenching stages having an unacceptably great length.
The U.S. Pat. No. 3,011,928 proposes a method for use with the conventional continuous wire-rolling mills which enables quenching stages of shorter length to be utilized. The wire, which leaves the rolling mill at 982.degree. C., is here initially cooled with water to between 733.degree. and 788.degree. C. and is subsequently coiled. The wire is further cooled to a temperature between 482.degree. and 705.degree. C. and, in particular, to a temperature between 533.degree. and 663.degree. C., in a period of about one minute by passing it through a misty atmosphere, that is, an atmosphere consisting of water vapor in air. This additional cooling is carried out either during passage of the wire to the spool or in the spool itself. This method, however, has the disadvantage that the individual windings of the wire in the coil undergo non-uniform cooling so that a consistent microstructure along the length of the wire cannot be insured.
It is, accordingly, a general object of the invention to provide a novel method of producing steel articles possessing drawing characteristics which are at least as good as those of the steel wires produced according to the prior art and which, at the same time, possess higher strength than the steel wires of the prior art.
A further object of the invention is to provide a process for making steel wires possessing good characteristics and strength by which process the articles may be produced more simply and economically than was possible heretofore and which requires less capital investment than the arrangements utilized until now.
It is also an object of the invention to provide a process for making steel wires possessing good drawing characteristics and strength by using shorter quenching stages than could be used heretofore.